Water-soluble polymeric dispersing and thickening agents of lower alkyl vinyl ether and maleic anhydride optionally including .alpha.-olefin monomer have been used in many latex and other formulations. However, high concentrations of these relatively low molecular weight polymers are required to provide improved results. In order to enhance their molecular weight and efficient use as thickeners, the polymers may be crosslinked with a polyfunctional compound, e.g. a C.sub.8 to C.sub.14 diene. It has been found that this innovation is not entirely satisfactory for, although crosslinking initially increases the viscosity, the salt tolerance of these crosslinked polymers is extremely low. Thus, upon addition to compositions containing a substantial amount of salt component, such as the ammonium and metal sulfates, citrates, chlorides, EDTA, etc. normally present in a concentration of about 1% in most cosmetic formulations, the polymer looses between 80 to 95% of its initial viscosity and fails to provide the desired thickening affect. Additionally, the salt containing polymeric compositions do not possess sufficient stability for storage and are subject to precipitation and phase separation. These affects are particularly troublesome when calcium or sodium chlorides are present in the composition.
Non-crosslinked alkyl vinyl ether, maleic anhydride and higher alkyl methacrylate terpolymers also have been employed as latex thickeners as shown in U.S. Pat. No. 3,436,378. However, these terpolymers are also subject to phase separation and, being of relatively low molecular weight and viscosity, do not provide a high degree of thickening. Additionally, these terpolymers do not form hydrogels.
Accordingly, it is an object of this invention to provide a crosslinked terpolymer hydrogel having high salt tolerance which retains up to 75% of its initial viscosity in salt containing compositions.
Another object of this invention is to provide a crosslinked terpolymer which is stable in compositions containing up to 1% salt.
Still another object is to provide a crosslinked terpolymer which is compatible with hard water and retains a viscosity of from about 20,000 to about 100,000 cps over extended periods of use or storage.
Yet another object is to provide a commercially feasible process for the preparation of a salt tolerant crosslinked terpolymer.